Power amplifiers (PAs), used in the amplification of communication signals, are inherently non-linear. Non-linear amplification of communication signals can lead to the signal entering adjacent radio frequencies and interfering with other communication signals. This can create challenges in complying with regulatory standards for spectral emissions.
A common method of improving the linearity of a transmit signal from a power amplifier is to use a digital pre-distortion (DPD) circuit. A DPD circuit inversely models the power amplifier's gain and phase characteristics to introduce an inverse distortion, prior to amplification, into the transmit signal. The inverse distortion cancels the power amplifier's distortion, but retains the amplification. A typical DPD circuit observes an amplified signal and compares it to the original transmit signal and makes adjustments until the amplified signal resembles the transmit signal such that it is free of distortions.
In a DPD circuit, the observed signal must propagate through an observation path to certain elements of the DPD circuit so that it can be compared to the transmit signal. The observation path must, accordingly, not distort the observed signal otherwise the DPD circuit will perceive the distortions as being caused by the power amplifier and will attempt to remove. In a DPD circuit capable of imparting frequency dependent compensation, it is typically required that the observation path also be essentially free of linear distortions such as frequency dependent gain and phase distortions.
FIG. 1 shows a typical DPD circuit 100 for a wireless communication transmitter. The DPD circuit 100 has a digital domain 102 and an analog domain 104. In the digital domain 102, a transmit signal Vm(t) to be amplified is generated by a modem 106 and sent along a transmit path Tx to a complex gain pre-distorter 108 and also an adaptive estimator 110. The adaptive estimator 110 compares the transmit signal Vm(t) to an observed signal Vo(t) which has been down-converted and digitized. Based on the adaptive estimator's 110 comparison, the complex gain pre-distorter 108 introduces an inverse distortion into the transmit signal Vm(t) to produce a pre-distorted signal Vd(t). The inverse distortion also compensates for the “loop” response (both the linear and non-linear response) occurring in the analog domain 104 as discussed below.
The pre-distorted signal Vd(t) is converted from digital to analog by a digital-to-analog-converter (DAC) 112, and then propagated to a baseband-to-RF up-converter 114. The baseband-to-RF up-converter 114 is controlled by a first local oscillator (LO) 116. The baseband-to-RF up-converter 114 outputs a RF signal to a power amplifier 118. The power amplifier 118 outputs an amplified signal Va(t) to an antenna 120 for wireless transmission. A directional coupler 122, observes the amplified signal Va(t) and outputs an observed signal Vo(t). The observed signal Vo(t) is received by the RF-to-baseband down-converter 124. The RF-to-baseband down-converter 124 is controlled by a second LO 126. The observed signal Vo(t) is converted back to a digital signal with an analog-to-digital-converter (ADC) 128 and input into the adaptive estimator 110.
The down-converted digitized observed signal Vo(t) contains a loop response, namely, linear and non-linear distortions introduced by the DAC 112, the baseband-to-RF up-converter 114, the power amplifier 118, the directional coupler 122, the RF-to-baseband down-converter 124, and the ADC 128. The adaptive estimator 110 adjusts the pre-distorter 108 to compensate for the loop response. Compensation is desired for the distortions introduced by the baseband-to-RF up-converter 114 and the power amplifier 118. These distortions occur in the transmit path Tx. Compensation is not desired, however, for the linear and non-linear distortions introduced in the observation path OP by, namely, the directional coupler 122, the RF-to-baseband down-converter 124, and the ADC 128. This undesired compensation is essentially equivalent to introducing an additional complementary response at the input of the pre-distorter 108.
Additional complementary response at the input of the pre-distorter degrades the quality of the transmit signal as measured by an Error Vector Magnitude or other suitable measurement procedures. For example, the presence of a gain slope across frequencies in the complementary linear response at the input of the pre-distorter 108 will result in a different output power level for each of the frequency channels within a multi-carrier signal bandwidth. This can have a detrimental effect on the performance of a pre-distortion system if the locations of the active signal carriers are dynamically changing within the multi-carrier signal bandwidth.